Calibration of in-line monitoring instruments in a subsea installation

ABSTRACT

A method of calibration of an in-line monitoring instrument in a subsea installation ( 6 ), comprises using an umbilical ( 4 ) for transportation of a sample fluid from the subsea installation ( 6 ) to a topside installation ( 2 ) and/or for transportation of a reference fluid from the topside installation ( 2 ) to the subsea installation ( 6 ); comparing measurements of the sample fluid or the reference fluid by the in-line monitoring instrument with measurements of the same fluid obtained previously or subsequently by topside instruments; and adjusting the calibration of the in-line monitoring instrument in the subsea installation ( 6 ), if required, based on the results of the comparison of measurements.

The invention relates to a method and an apparatus for calibration of anin-line monitoring instrument in a subsea installation. In someexamples, the in-line monitoring instruments are for measurement of datarelating to production in an oil and gas installation, such as fiscalmetering for an oil and gas production installation.

In the oil and gas industry, amongst other industries, there is a needfor obtaining information regarding subsea installations and the fluidsthat are passed through such installations. It will be appreciated thatthere are various challenges connected with this, due to the location ofthe subsea installation and since the fluids concerned are naturallyproduced fluids with unpredictable characteristics. One particularexample is the need to monitor production fluids for an oil and gasproduction installation, and to obtain in-line measurements such aswater fraction (also denoted water cut), gas to oil ratio and the likein order to allow production to be measured and potentially optimised,and in order for fiscal metering/hydrocarbon accounting to be carriedout. An installation might include multiple production lines far belowthe surface of the sea, with the lines combining before they reach a topside installation. For effective monitoring it is necessary to carry outmeasurements in line with the subsea pipelines, and this hence requiresinstruments to be mounted in remote locations below the surface of thesea.

In this context, a problem arises since, as with any monitoringequipment, periodic calibration is required for the monitoringinstruments that are mounted in the subsea pipelines. In a priorpublication by the inventor, “A method for remote in-line calibration ofwater fraction meters” by Dag Flolo, presented at 25th InternationalNorth Sea Flow Measurement Workshop 2007 and available athttp://www.linknovate.com/publication/a-method-for-remote-in-line-calibration-of-water-fraction-meters-763441/,the issues relating to remote in-line calibration in the context ofwater fraction meters are discussed and a method for remote in-linecalibration is proposed that involves settling of water in a verticalline under no flow conditions.

The proposal involves the use of new metering stations with variouslimitations compared to existing metering stations, for example theability to remotely access the metering station and water fractionmeter, the ability to ensure that shutdowns with no flow conditions andwithout depressurisation can occur (or the redesign of valvearrangements so that no flow conditions can be created withoutshutdown), amongst other things. Metering station with the ability forremote in-line calibration as described in the prior publication wouldprovide great benefits, but as yet this does not exist in a practicalform.

Viewed from a first aspect, the present invention provides a method ofcalibration of an in-line monitoring instrument in a subseainstallation, the method comprising: using an umbilical fortransportation of a sample fluid from the subsea installation to atopside installation and/or for transportation of a reference fluid fromthe topside installation to the subsea installation; comparingmeasurements of the sample fluid or the reference fluid by the in-linemonitoring instrument with measurements of the same fluid obtainedpreviously or subsequently by topside instruments; and adjusting thecalibration of the in-line monitoring instrument in the subseainstallation, if required, based on the results of the comparison ofmeasurements.

The inventor has made the realisation that problems with subseacalibration can be resolved by the use of an umbilical for physicaltransportation of fluids between the subsea installation and a topsideinstallation. In many cases an umbilical will already be present, andtherefore all that is required is that the umbilical is provided with asuitable fluid transfer mechanism and appropriate attachments to enableliquid to be extracted or introduced into the in-line monitoringinstrument at the subsea installation.

In the case where a sample fluid is used then the sample fluid can beobtained from the subsea installation, transported up the umbilical tothe topside installation, and passed to a suitable topside instrumentfor measurements to be taken. A measurement of the sample fluid or ofequivalent fluid may be taken by the in-line monitoring instrument ataround the same time that the sample is obtained, for example within aspecific time period and from the same fluid source.

The method may include measuring a sample of the fluid using the in-linemonitoring instrument at the subsea installation, and then transportingthe exact same fluid to the topside installation for the comparativemeasurement to be taken. This latter approach means that there is norisk of change in the parameters of the fluid flowing through the subseainstallation between the time that the sample is taken and thecomparative measurement is taken at the subsea installation. After thetopside measurements have been taken and compared with the subseameasurements then calibration of the in-line monitoring device can beadjusted based on a difference between the two measurements.

Alternatively, fluid measured by the in-line monitoring instrument inthe subsea installation may be sampled at a later point, after it haspassed through the in-line monitoring instrument. The sampling locationand the timing of the sample may be set so as to ensure that the fluidwhich is sampled is the same as or very similar to the fluid measured bythe in-line monitoring instrument. In this case there will be a greaterdelay between the initial measurement by the in-line monitoringinstrument and the comparative measurement at the topside installation,and the calibration of the in-line monitoring instrument can be adjustedretrospectively based on comparison of the measurements.

In another alternative the sample fluid may be extracted upstream of thein-line monitoring instrument, i.e. before the working fluid has reachedthe in-line monitoring instrument. The sample fluid is measured at thetopside installation after it has been passed upward through theumbilical, and the in-line monitoring instrument measures the workingfluid at a later point. The timing and location of extraction of thesample is known and can be used to determine when the appropriate partof the working fluid will reach the in-line monitoring instrument.Calibration of the in-line monitoring instrument could be adjustedduring the measurement in order to ensure that the two readings match,or again the calibration could be adjusted retrospectively.

In the case where a reference fluid is used then the reference fluid canbe first measured using a suitable topside instrument, then transporteddown the umbilical to the subsea installation, and introduced into thein-line monitoring instrument at the subsea installation to therebyenable a measurement of the reference fluid to be obtained forcomparison purposes. In this case the two measurements will alwaysrelate to the same fluid. The reference fluid could be a specificallydesigned calibration fluid, or it may simply be a sample with knownqualities where an accurate measurement can be taken by the topsideinstrument for comparison with the subsequent measurement by the subseainstallation instrumentation. In this case calibration of the in-linemonitoring instrument could occur retrospectively, after themeasurements, or as for the upstream sampling option mentioned above thecalibration could occur simultaneous with measurement of the referencefluid in order to ensure that the measurement is consistent with themeasurement from the topside instrumentation.

The method may be utilised with a single sample fluid or referencefluid, and the calibration step carried out for a single set ofmeasurements then optionally repeated for further single fluids.Alternatively the method may make use of multiple samples taken over aperiod of time in order to obtain an average of the fluid parameter fora given volume of the working fluid, with calibration then being basedon the average values. Combinations of these two possibilities may alsobe used in order to further optimise calibration of the in-linemonitoring instruments.

The measurements taken by the in-line monitoring instrument can be anytype of measurement dependent on the nature of the subsea installation.They may for example be measurement of fluid parameters concerning waterfraction, density, viscosity, permittivity, conductivity, gas-oil ratio,gas quality measurements, gas composition, and/or water qualitymeasurements. In some examples the measurements are of the type takenfor fiscal metering/hydrocarbon accounting.

The method may involve calibration of multiple in-line monitoringinstruments, with the umbilical being used for transportation ofmultiple sample fluids or reference fluids, each being relevant to adifferent monitoring instrument. The calibration step may use anysuitable technique for the monitoring instrument in question. In somecases the calibration will be done by a human operator adjustingappropriate settings based on the measured data or based on a comparisonof the data produced by a measuring device and/or by a data processingdevice such as a computer. In other cases there may be automatedcalibration through remote adjustment of the in-line monitoringinstrument using a data processing device such as a computer. The dataprocessing device may be arranged to communicate with the in-linemonitoring device via the umbilical.

Transportation of the fluid through the umbilical may be propelled bythe pressure already present at the subsea installation when fluid isbeing transported upward to the topside installation, and/or may beassisted by pumping. Transportation of the fluid downward through theumbilical would typically require pumping in order to overcome thepressure difference between topside installation and the subseainstallation. The umbilical is preferably provided with sample lines forthe purpose of transporting the sample or reference fluid. The samplelines may take the form of heated tube bundles. When multiple in-linemonitoring instruments are being calibrated then separate sample linesmay be used for fluids for separate instruments. Typically the innerdiameter of the sample lines will be in the range 6 to 20 mm. The flowvelocity may be in the range 0.5 to 60 m/s.

Viewed from a second aspect the invention provides a system forcalibration of an in-line monitoring instrument in a subseainstallation, the system comprising: an umbilical equipped with a sampleline for transporting fluid between a subsea installation and a topsideinstallation; a subsea coupling for connecting the sample line to thesubsea installation and enabling extraction or introduction of fluidfrom or to the subsea installation; a topside coupling for introductionof fluid to the sample line or extraction of fluid from the sample line;and one or more measurement device(s) for receiving one or moremeasurement(s) of the fluid from a topside instrument, receiving one ormore measurement(s) of the same or a similar fluid from the in-linemonitoring instrument and providing data enabling calibration of thein-line monitoring instrument based on a comparison of the measurements.

The system of this aspect may be used with a sample fluid or referencefluid as described above in connection with the first aspect. Thus, asample fluid can be obtained from the subsea installation andtransported up the umbilical to the topside installation, or a referencefluid could be transported down the umbilical from the topsideinstallation to the subsea installation. Where a reference fluid is usedthen the in-line monitoring instrument will be able to measure thereference fluid and hence take measurements of the same fluid that ismeasured topside, and where a sample fluid is used then the in-linemonitoring instrument may take a measurement of the same fluid, or of asimilar fluid upstream or downstream of the sampling point.

The measurement device(s) may be arranged to determine an appropriatetiming for a measurement by the in-line monitoring instrument and/or anappropriate timing for a sample to be extracted upstream or downstreamin the fluid flow through the in-line monitoring instrument. The timingmay, for example, be determined based on the relative locations of thein-line monitoring instrument and the location of the subsea coupling,along with information regarding flow speeds between the two points,which could be based on known operating parameters, or on measured flowrates.

The measurement device(s) may be arranged to provide appropriate data toenable a human operator to compare measurements and determine anappropriate adjustment to calibrate the in-line monitoring instrument.Alternatively, the measurement device(s) may have the ability to comparedata and optionally to propose an adjustment for calibration of thein-line monitoring instrument. For example the measurement device(s) maybe embodied by a data processing device such as a computer. There may bean automatic calibration device for communications with the in-linemonitoring instrument and automatic remote calibration thereof, based ondata from the measurement device(s). Advantageously the calibrationdevice may communicate with the in-line monitoring instrument via theumbilical. In some examples there are multiple measurement devices witha first device communicating with, or provided as a part of, the in-linemonitoring instrument to receive measurements therefrom and to providedata indicative of the fluid properties as measured at the in-linemonitoring instrument, and a second device communicating with, orprovided as a part of, the topside instrument to receive measurementstherefrom and to provide data indicative of the fluid properties asmeasured at the topside instrument.

The in-line monitoring instrument can be any type ofmeasurement/metering instrument. It may for example be an instrument formeasurement of fluid parameters concerning water fraction, density,viscosity, permittivity, conductivity, gas-oil ratio, gas qualitymeasurements, gas composition, and/or water quality measurements. Insome examples the measurements are of the type taken for fiscalmetering/hydrocarbon accounting.

There may be multiple in-line monitoring instruments, with the umbilicalbeing arranged for transportation of multiple fluids each being relevantto a different monitoring instrument.

A pump or pumps may be included in the system for transportation of thefluid through the umbilical. The system may include one or more valvesor the like for control of flow, and the measurement calibration device,or alternatively a separate control system, may be arranged forcontrolling the valves and optionally the pumps if they are included inorder to achieve the desired flow of the fluid through the umbilical.

The umbilical is provided with sample lines for the purpose oftransporting the sample or reference fluid. The sample lines may takethe form of heated tube bundles. When multiple in-line monitoringinstruments are being calibrated then separate sample lines may be usedfor fluids for separate instruments. Typically the inner diameter of thesample lines will be in the range 6 to 20 mm. The flow velocity may bein the range 0.5 to 60 m/s.

The umbilical may further include control lines for control of any ofthe various parts of the system (for example, the couplings, the in-linemonitoring instrument and calibration thereof, valves, pumps and so on).The control lines may be of any suitable type such as control lines useconventionally in umbilical is for similar purposes, and mightconveniently be control lines for electrical signals.

A preferred embodiment of the invention will now be described by way ofexample only and with reference to the accompanying FIGURE in which:

FIG. 1 illustrates connections between a subsea installation and atopside installation.

As shown in the FIGURE a host topside installation 2 can be provided inthe form of an oil and gas production installation equipped with theability for various analysis and monitoring tasks to be carried out. Thetopside installation 2 is connected via an umbilical 4 to a subseainstallation 6. The subsea installation includes one or more in-linemonitoring instruments connected to pipelines for working fluids such ashydrocarbon fluids. The in-line monitoring instruments are arranged forongoing measurement of fluid parameters of the working fluids, forexample measurements as used for fiscal metering.

The host topside installation 2 can, in particular, include suitablemeasurement instruments for providing a qualified accurate measurementof a sample fluid or reference fluid to act as comparison measurementsfor measurements taken by the in-line monitoring instruments.

The umbilical 4 can include typical known features for an umbilical usedto couple between the subsea installation 6 and the topside installation2, and it is further provided with sampling lines in the form of heatedtube bundles within the umbilical for transportation of sample orreference fluids between the subsea installation 6 and the topsideinstallation 2.

At the topside end of the umbilical there is a topside coupling arrangedto permit fluid to be introduced into the sample line at the topsideand, or removed from the sample line at the topside end. The fluidremoved at the topside end may be passed to an instrument in the samelocation, via tubing or by human transfer of a sample vessel.Alternatively the fluid may be transported to a topside instrument at adifferent location, remote from the topside end of the umbilical, forexample an on-shore laboratory.

At the subsea end of the umbilical there is a subsea coupling arrangedto permit fluid to be introduced or removed at the subsea end. Thesubsea coupling can be attached directly to the in-line monitoringinstrument, enabling fluid to be passed between the two and thereforeensuring that identical fluid is measured at the in-line measuringinstrument and is conveyed along the umbilical. Alternatively, thesubsea coupling can be attached to a pipeline upstream or downstream ofthe in-line monitoring instrument, for upstream or downstream samplingas discussed above.

Measurements from the topside instrument and the in-line monitoringinstrument can be taken using one or more measurement device. Themeasurement device(s) may output data representative of the fluidparameters as measured by the respective instrument, and this data canbe used by a human operator to compare the performance of the in-linemonitoring instrument with the topside instrument and to determine anappropriate adjustment for calibration of the in-line monitoringinstrument. Alternatively, an automatic measurement and calibrationsystem may be included, and could typically be located at the topsideinstallation 2, although it could be remotely located. The measurementand calibration system might include measurement devices connected to orformed as a part of a data processing system, with the data processingsystem also having the capability to compare measurements and determinea suitable adjustment for calibration of the in-line monitoringinstrument. A measurement and calibration system can thus be arranged toreceive measurements taken by the in-line monitoring instrument at thesubsea installation 6 and measurements taken by the topside measuringinstruments. The measurement and calibration system can then compare themeasurements from subsea and topside in order to check the calibrationof the in-line monitoring instrument. If the comparison reveals anydiscrepancies then the calibration of the in-line monitoring instrumentis adjusted, for example via adjustment of operating parameters of thein-line monitoring instrument by the measurement and calibration systemusing an automated adjustment system, which can advantageouslycommunicate with the in-line monitoring system via the umbilical.

1-27. (canceled)
 28. A method of calibration of an in-line monitoringinstrument in a subsea installation, the method comprising: using anumbilical for transportation of a sample fluid from the subseainstallation to a topside installation and/or for transportation of areference fluid from the topside installation to the subseainstallation; comparing measurements of the sample fluid or thereference fluid by the in-line monitoring instrument with measurementsof the same fluid obtained previously or subsequently by topsideinstruments; and adjusting the calibration of the in-line monitoringinstrument in the subsea installation, if required, based on the resultsof the comparison of measurements.
 29. A method as claimed in claim 28,wherein a sample fluid is used and the sample fluid is obtained from thesubsea installation, transported up the umbilical to the topsideinstallation, and passed to the topside instrument for measurements tobe taken.
 30. A method as claimed in claim 29, wherein a measurement ofthe sample fluid or of equivalent fluid is taken by the in-linemonitoring instrument at around the same time that the sample isobtained.
 31. A method as claimed in claim 29, comprising: measuring asample of the fluid using the in-line monitoring instrument at thesubsea installation, and then transporting the exact same fluid to thetopside installation for the comparative measurement to be taken.
 32. Amethod as claimed in claim 29, wherein sample fluid is first measured bythe in-line monitoring instrument in the subsea installation and thensampled at a later point, after the fluid has passed through the in-linemonitoring instrument.
 33. A method as claimed in claim 28, wherein thesample fluid is extracted upstream of the in-line monitoring instrument,the sample fluid is measured at the topside installation after it hasbeen passed upward through the umbilical, and the in-line monitoringinstrument measures the same working fluid at a later point.
 34. Amethod as claimed in claim 28, wherein a reference fluid is used and thereference fluid is first measured using a suitable topside instrument,then transported down the umbilical to the subsea installation, andintroduced into the in-line monitoring instrument at the subseainstallation to thereby enable a measurement of the reference fluid tobe obtained for comparison purposes.
 35. A method as claimed in claim28, being used for calibration of multiple in-line monitoringinstruments, with the umbilical being used for transportation ofmultiple sample fluids or reference fluids, each being relevant to adifferent monitoring instrument.
 36. A method as claimed in claim 28,wherein the calibrating step is carried out by a human operatoradjusting appropriate settings based on the measured data or based on acomparison of the data produced by the measuring device(s) and/or by adata processing device such as a computer.
 37. A method as claimed inclaim 28, wherein the calibrating step comprises automated calibrationthrough remote adjustment of the in-line monitoring instrument using adata processing device.
 38. A method as claimed in claim 28, wherein theumbilical is provided with one or more sample line(s) for the purpose oftransporting the sample or reference fluid, and wherein the sample linestake the form of heated tube bundles.
 39. A method as claimed in claim38, wherein the inner diameter of the sample line(s) is in the range 6to 20 mm and/or the flow velocity is in the range 0.5 to 60 m/s.
 40. Asystem for calibration of an in-line monitoring instrument in a subseainstallation, the system comprising: an umbilical equipped with a sampleline for transporting fluid between a subsea installation and a topsideinstallation; a subsea coupling for connecting the sample line to thesubsea installation and enabling extraction or introduction of fluidfrom or to the subsea installation; a topside coupling for introductionof fluid to the sample line or extraction of fluid from the sample line;and one or more measurement device(s) for receiving one or moremeasurement(s) of the fluid from a topside instrument, receiving one ormore measurement(s) of the same or a similar fluid from the in-linemonitoring instrument and providing data enabling calibration of thein-line monitoring instrument based on a comparison of the measurements.41. A system as claimed in claim 40, wherein the measurement device(s)are arranged to provide appropriate data to enable a human operator tocompare measurements and determine an appropriate adjustment tocalibrate the in-line monitoring instrument.
 42. A system as claimed inclaim 40, wherein the measurement device(s) are arranged to compare dataand optionally to propose an adjustment for calibration of the in-linemonitoring instrument.
 43. A system as claimed in claim 40, comprisingan automatic calibration device for communications with the in-linemonitoring instrument and automatic remote calibration thereof, based ondata from the measurement device(s).
 44. A system as claimed in claim40, wherein the calibration device communicates with the in-linemonitoring instrument via the umbilical.
 45. A system as claimed inclaim 40, wherein the system is for calibration of multiple in-linemonitoring instruments, with the umbilical hence being arranged fortransportation of multiple fluids each being relevant to a differentmonitoring instrument.
 46. A system as claimed in claim 40, wherein theumbilical is provided with one or more sample lines for the purpose oftransporting the sample fluid or reference fluid, and wherein the sampleline(s) are heated tube bundles.
 47. A system as claimed in claim 46,wherein the inner diameter of the sample lines is in the range 6 to 20mm and/or the flow velocity is in the range 0.5 to 60 m/s.